devtool: add mechanism for updating extensible SDK

Enable updating the installed extensible SDK from a local or remote
server, avoiding the need to install it again from scratch when
updating. (This assumes that the updated SDK has been built and then
published somewhere using the oe-publish-sdk script beforehand.)

This plugin is only enabled when devtool is used within the extensible
SDK since it doesn't make sense to use it next to a normal install of
the build system.

E.g.
devtool sdk-update /mnt/sdk-repo/
devtool sdk-update http://mysdkhost/sdk

(From OE-Core rev: 32cbd4c57fc8ca097a18929fc404c07322ef36dd)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/populate_sdk_ext.bbclass

@@ -18,6 +18,7 @@ SDK_RELOCATE_AFTER_INSTALL_task-populate-sdk-ext = "0"
 SDK_LOCAL_CONF_WHITELIST ?= ""
 SDK_LOCAL_CONF_BLACKLIST ?= "CONF_VERSION BB_NUMBER_THREADS PARALLEL_MAKE PRSERV_HOST"
 SDK_INHERIT_BLACKLIST ?= "buildhistory icecc"
+SDK_UPDATE_URL ?= ""
 
 SDK_TARGETS ?= "${PN}"
 OE_INIT_ENV_SCRIPT ?= "oe-init-build-env"
@@ -85,6 +86,11 @@ python copy_buildsystem () {
     config.set('General', 'bitbake_subdir', conf_bbpath)
     config.set('General', 'init_path', conf_initpath)
     config.set('General', 'core_meta_subdir', core_meta_subdir)
+    config.add_section('SDK')
+    config.set('SDK', 'sdk_targets', d.getVar('SDK_TARGETS', True))
+    updateurl = d.getVar('SDK_UPDATE_URL', True)
+    if updateurl:
+        config.set('SDK', 'updateserver', updateurl)
     bb.utils.mkdirhier(os.path.join(baseoutpath, 'conf'))
     with open(os.path.join(baseoutpath, 'conf', 'devtool.conf'), 'w') as f:
         config.write(f)